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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        let arr = new Array(20).fill(0).map((_,index) => index+1);//undefined-0-n 
                                           //_表示当前元素。。。
                for(let i=0;i<arr.length;i++)
                {
                    console.log(arr[i]);
                }
                //arr名字不对应？
    </script>
</body>
</html>